Failure detection circuit

ABSTRACT

A circuit for detecting the failure of one or more of a plurality of incandescent lamps or other electrical devices which are subject to failure. Each device is connected to a power source such that a predetermined low current flows through each good device. The current in each device is normally appreciably lower than the current required to energize the device. A circuit including an isolation diode also connects each device to a common terminal. The voltage on the common terminal changes in the event of a failure of any of the devices causing a current decrease in such device. A voltage change on the common terminal operates an electronic switch to energize an alarm or indicator, such as a warning lamp.

United States Patent 1191 Doyle March 6, 1973 [54] FAILURE DETECTIONCIRCUIT 3,311,779 3/1967 Hartkorn, Jr ..340/256 x Inventor: James H. y gCalif. 3,604,949 9/l97l Conzelmann et al. ..340/25l X [73] Assignee:Master Specialties Company, Costa Primary Examiner-John W. CaldwellMesa, Calif. Assistant Examiner-Scott F. Partridge 22 Filed: Feb.11,1971

[2]] Appl. No.: 114,580 [57] ABSTRACT Related s Appncation Dam A circuitfor detecting the failure of one or more of a plurahty of incandescentlamps or other electrical [63] commuaflon'm'pa of 441122 June deviceswhich are subject to failure. Each device is 1970 abandoned connected toa power source such that a predetermined low current flows through eachgood device. [52] U.S. Cl. ..340/25l, 340/256, 340/336 The current ineach device is normally appreciably [51] lllt. Cl. lower than thecurrent required to energize the device [58] Fleld of Search"34O/248 5 Acircuit including an isolation diode also connects 340/253 336; 5 136each device to a common terminal. The voltage on the g common terminalchanges in the event of a failure of [56] References cued any of thedevices causing a current decrease in such UNITED STATES PATENTS device.A voltage change on the common terminal operates an electromc switch toenergize an alarm or 2,963,692 12/1960 Barter et al. ..340/336 UXindicator, such as a warning lamp. 3,505,664 4/1970 Morris 1 3,271,7369/1966 Brown et al. ..340/253 X 12 Claims, 3 Drawing FiguresPATENTEUMXR' 61975 3,719,937

INVENVTOR." JA ES HD/J L @QQA ,HTTT/SI FAILURE DETECTION cmcurTCROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-part ofmy copending application Ser. No. 44,122, filed June 8, 1970, nowabandoned.

BACKGROUND OF THE INVENTION This invention relates to detector circuitsand, more particularly, to a circuit for detecting the failure of one ormore of a plurality of incandescent lamps or other electrical deviceswhich are subject to failure.

Incandescent lamps and other light sources are commonly used as a sourceof information. In aircraft, for example, an annunciator panel ispositioned near the pilot. In the event of the occurrence of a monitoredcondition, such as low fuel, landing gear down, low oil pressure, etc.,an associated indicator lamp is energized to give the pilot a warning.If, however, a lamp fails, the pilot may not be notified of theoccurrence of a critical condition. Similarly, light sources arecommonly used in segment type readout devices found in variousinstruments. By energizing preselected combinations of the lightsources, the numbers zero through nine or other symbols can be formed.In devices of this type,

the failure of one or more of the light sources may result in anerroneous indication. If, for example, a segment readout device isenergized to indicate the number 7 and the upper light source fails, thereadout device will actually indicate the number 1. A false indicationof this type can lead to numerous problems, particularly since thereadout device appears to give a valid reading.

In the past, there has been no satisfactory circuit for simultaneouslyand continuously monitoring a plurality of indicator lamps, neon lamps,light emitting diodes, relays, or other similar devices. A plurality ofdevices of this type are generally tested by simultaneously energizingall of the devices and noting if any device has failed. However, such atest will not indicate the failure of a device after the test iscompleted.

SUMMARY OF THE INVENTION According to the instant invention, a circuitis provided for continuously monitoring a plurality of electricaldevices which are subject to failure, such as incandescent lamps, neonlamps, light emitting diodes, relays and the like, to detect the failureof one or more of the devices. In the event of a failure, a warninglamp, a relay, or some other suitable alarm is energized. The circuit isdesigned to cause a referencecurrent to continuously flow through eachdevice. In one embodiment, the current also flows through a seriesresistor which is selected to limit the reference current to a levelappreciably lower than the current required to energize the device. Thedevice is energized by closing a switch to short out the seriesresistor. In the event of a failure of one or more of the devices, thecurrent in the failed device decreases below the reference value. Acurrent decrease in any device to less than the reference value issensed by an electronic switching circuit which energizes the alarm.

In a second embodiment, each device is connected in parallel to a powersource. A diode having a predetermined forward voltage drop and atransistor switch are connected in series with each device and the powersource. Each device is energized by closing an associated one of thetransistor switches. The junction between each device and the seriesdiode is connected to the control element of an electronic switch whichsenses a decrease in a minimum reference current through the device andsuch control element. The electronic switch can be connected to energizean alarm or to energize all of the devices which have not failed,thereby indicating the location of any failures.

Accordingly, it is a primary object of the invention to provide acircuit for continuously monitoring a plurality of electricallyconducting devices for detecting the failure ofone or more of suchdevices.

Another object of the invention is to provide a circuit for monitoringand detecting the failure of one or more lamps in a device having aplurality ofindicator lamps.

Other objects and advantages of the invention will become apparent fromthe following detailed description, with reference being made to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevational view ofa segment type readout device;

FIG. 2 is a detailed schematic circuit diagram ofa detector circuitconstructed in accordance with one preferred embodiment of the presentinvention; and

FIG. 3 is a detailed schematic circuit diagram of a detector circuitconstructed in accordance with a second preferred embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the presentinvention, a circuit is provided for detecting and indicating thefailure of one or more of a plurality of electrical devices. The devicesmay be, for example, light sources, such as incandescent lamps, neonlamps and light emitting diodes, or relay windings and the like. In thefollowing detailed description, two circuits are described connected formonitoring a plurality of incandescent lamps in a segment type readoutdevice. It is, of course, obvious that the circuits may be used tomonitor other electrical devices and, therefore, it is not intended tolimit the circuits to this particular use.

Referring now to FIG. 1, the front of a typical seven segment readoutdevice 10 is shown in detail. The readout device 10 is designed to mountin the front of a panel, such as a computer control panel or on thefront of a direct reading voltmeter, for selectively indicating anynumber zero through nine. The readout device 10 is provided with eightincandescent lamps, 11 through 18, for illuminating the seven segmentsand a decimal point, respectively. Various numbers are formed byenergizing combinations of the lamps, forexample, the number 1 is formedby energizing the lamps l2 and 13 and the number 7 is formed byenergizing the lamps l 1, l2 and 13. It can be seen that if the lamp 11fails, the number 7 will erroneously appear as the number 1. Thus, ifthe readout device has no lamp failure indicator, an instrument operatormay rely upon erroneous data.

Turning to FIG. 2, a circuit 20 is shown connected to the eight lamps 11through 18 for detecting the failure of any one or more of the lamps. Asuitable power source is connected between a positive terminal 21 and anegative ground terminal 22. For the purposes of this discussion, itwill be assumed that the power source is a five volt direct currentsource, although it is not intended that the invention be limited to apower source of this type or voltage. It will also be assumed that thelamps 11 through 18 each require three to five volts at twenty to fiftymilliamperes for operation.

The lamp 11 is connected through a resistor 23 to the positive terminal21 and through a current limiting resistor 24 to the ground terminal 22.The lamps 12 through 18 are each similarly connected to the positiveterminal 21 and the ground terminal 22, with corresponding resistorslabeled 23' and 24. The value of the resistor 24 is selected to limitthe current through the lamp 11 to a value appreciably below the currentrequired to energize the lamp 11, for example, one or two milliamperes.

A diode 25 and a diode 26 are connected in series between the positiveterminal 21 and a terminal 27. The terminal 27 isthen connected througha diode 28 to the lamp 11, and similarly through a plurality of paralleldiodes 28' to the lamps 12 through 18. Thus, the series diodes 25, 26and 28 are in parallel with the resistor 23. The value of the resistor23 is selected to be such that the current flowing through it will beless than the current through the resistor 24, so that the diode 28 willprovide a clamping function. A switch 29 is provided in parallel withthe resistor 24 for selectively energizing the lamp 11. When the switch29 is closed, the resistor 24 is shorted and one side of the lamp 11 isconnected directly to the ground terminal 22. Current for operating thelamp 11 is then provided through the series diodes 25,26 and 28. Thevoltage applied to the lamp 11 will equal the voltage applied betweenthe terminals 21 and 22 less the forward voltage drop of the threediodes 25, 26 and 28. Thus, if the diodes have a forward voltage drop of0.6 volts, 3.2 volts will appear on the lamp 11 when the switch 29 isclosed and the lamp 11 will be energized. When the switch is open orclosed the voltage at the top of the lamp will remain approximately 0.6volts below the voltage on the line 27 (due to the clamping action ofthe diode 28).

A solid state switching circuit including a pair of PNP transistors 31and 32 is also connected between the positive terminal 21 and the groundterminal 22. The emitter of the transistor 31 is connected to thejunction between the series diodes 25 and 26, which is at a voltageequal to the voltage on the line 27 plus the forward voltage drop of thediode 26. The collector of the transistor 31 is connected through aresistor 33 to the ground terminal 22 and the base is connected througha resistor 34 to the ground terminal 22. The base of the transistor 31is also connected through a diode 35 to the common junction between thelamp 11, the diode 28 and the resistor 23. Similarly, the base of thetransistor 31 is connected through a plurality of diodes 35' to thelamps 12 through 18. The diodes 35 and 35' form an AND-circuit forapplying a current'to the base of the transistor 31. The transistor 31is normally in the on state since the current flowing through theresistor 34 is flowing into its base. The resistor 23 is of a value tocause a current flow greater than the current flowing through theresistor 34. The current flowing through the resistor 23 normally has asits ground path the light 11 and resistor 24. In this condition thediode 35 is back biased preventing current to flow into the resistor 34.When light 11, or any other light 12 through 18, fails, all of thecurrent flowing through the resistor 23 flows through the diode 35 intothe resistor 34. Since the current flowing through the resistor 23 isnormally greater than the current flowing through the resistor 34 all ofthe'current through the resistor 34 is diverted away from the base ofthe transistor 31, thereby turning it off. This current is generated bythe voltage drop across the resistor 34 (approximately 3.5 volts). Inthe event of a failure of any of the lamps, such as lamp 11, the currentthrough resistor 34 is switched through the diode 35 and the resistor23, and does not flowinto the base of the transistor 31, thereby causingthe transistor 31 to go into a nonconducting state. For this gatingfunction to take place the current through the resistor 23 is greaterthan the current through the resistor 34.

The transistor 32 has an emitter connected to the terminal 27 and a baseconnected to the collector of the transistor 31. The collector of thetransistor 32 is con 'nected through a suitable alarm, such as a warninglamp 36, to the ground terminal 22. As long as the transistor 31conducts, the current flowing through the resistor 33 will pass throughthe transistor 31 and the base of transistor 32 will not have any drivecurrent, thus causing the transistor 32 to be in a nonconducting state.When the failure of one or more of the lamps 1 1 through 18 biases thetransistor 31 into a nonconducting state, the current flowing throughthe resistor 33 will flow into the base of the transistor 32, switchingthe transistor 32 into a conducting state. It is apparent that othertypes of alarms, such as an audible alarm or a relay, may be connectedeither in series with, in parallel with, or in place of the warning lamp36.

Turning now to FIG. 3, a second detector circuit embodiment 40 is shownin detail. As in FIG. 2, the detection circuit elements for one deviceare labeled with whole numbers and corresponding elements for otherdevices are labeled with corresponding prime numbers. In thisembodiment, a plurality of indicator lamps 41 are each connected to apositive terminal 42. Each lamp is also connected in series through adiode 43 and an NPN transistor 44 to a negative or ground terminal 45.The diode 43 may be a single diode or a series of diodes selected tohave a predetermined forward bias voltage drop. If, for example, aconventional 5-volt power source is connected between the positiveterminal 42 and the ground terminal 45, the diode 43 may be selected tohave a l.4-volt forward bias voltage drop. Whenever a transistor 44conducts, 3.6 volts will appear across the associated lamp 41 toilluminate the lamp. The positive terminal 42 is also connected througha switch 46 and an OR gate 47 to the base of the transistor 44. When theswitch 46 is closed, the gate 47 applies a positive voltage to the baseof the transistor 44, causing the transistor 44 to conduct to illuminatethe lamp 41.

A junction 48 between the lamp 41 and the diode 43 is connected througha resistor 49 to the base of an NPN transistor 50. The collector of thetransistor 50 is connected through a resistor 51 to the positiveterminal 42, while the emitter is connected to the ground terminal 45,.Three distinct voltages may appear on the terminal 48: approximately 5volts will appear on the terminal 48 when the transistor 44 isnonconducting and the lamp 41 is good; 1.4 volts plus the forwardvoltage drop of the transistor 44 will appear on the terminal 48 whilethe transistor 44 is conducting and the lamp 41 is illuminated; and zerovolts will appear upon the terminal 44 in the event of a failure of thelamp 41. So long as at least 1.4 volts appear on the terminal 48, thetransistor 50 is biased into a conducting state by a low current flowingthrough the lamp 41, the resistor 49 and the base of the transistor 50.A junction between the resistor 51 and the collector of the transistor50 will be near the potential on the ground 45 when the transistor 50conducts. All of the current flowing through the resistor 51 will flowthrough the transistor 50 to the ground 45. The terminals 52 and 52 foreach transistor 50 and 50 are connected through an isolation diode 53 or53' to a common terminal 54. When a lamp failure causes the transistor50 to stop conducting, the current flowing through the resistor 51 flowsthrough a diode 53, the terminal 54 and the base of a transistor 55. Aslong as all of the transistors 50 and 50 conduct, no current will flowthrough the terminal 54. However, in the event ofa failure of one ormore of the lamps 41 or 41' causing one or more of the transistors 50 or50 to stop conducting, the current flowing through the resistors 51 or51' will be applied through one or more of the diodes 53 or 53 to thecommon terminal 54 and the base of the transistor 55.

The common terminal 54 is connected to the base of an NPN transistor 55and through a bias resistor 56 to the ground terminal 45. The collectorof the transistor 55 is connected through a resistor 57 to the positiveterminal 42 and to the input of a conventional inverteramplifier 58. Theamplifier 58 will have a high output whenever the terminal 54 is passingcurrent due to a failure of one or more of the lamps 41 and 41 Theoutput of the amplifier 58 is connected through a switch 59 to either analarm, such as an incandescent lamp 60, or to a buss 61. When the switch59 connects the output of the amplifier 58 to the lamp 60, the lamp 60will be illuminated in the event of a failure of one or more of thelamps 41 and 41'. The buss 61 is connected in parallel to a second inputof each of the OR gates 47 and 47. When the output of the amplifier 58is connected to the buss 61 and one or more of the lamps 41 or 41'fails, all good lamps 41 and 41' will be illuminated. Thus, any of thelamps which fail may be readily located. In a modified embodiment, theswitch 59 can be eliminated and the output of the amplifier 58 connecteddirectly to the buss 61. In this case, a failure of any one of the lamps41 and 41 will automatically illuminate all of the good lamps. Thus, thelamps 41 and 41 form an alarm to indicate both the failure of one ofsuch lamps and the location of the failure.

It will be appreciated that the values of the various circuit componentsmay be selected such that the alarm is energized when the current in anyof the monitored lamps or other monitored devices decreases belowapreselected level. The components of FIG. 2 may, for example, beselected to energize the alarm in the event that the current in any lampdrops below one milliampere. This current may be considerably lower withthe circuit of FIG. 3. Should any lamp or the connection to any lamphave an abnormally high resistance, the circuit will operate to give analarm even though there is not a total failure of the lamp. It will alsobe appreciated that other arrangements of the detector circuit may beused and that the detector circuit may be used for monitoring any numberof electrical devices other than incandescent lamps without departingfrom the spirit and the scope of the appended claims.

What I claim is:

1. A circuit for detecting the electrical failure of any of a pluralityof electrical devices each of which requires a predetermined current tooperate comprising, in combination, a plurality of resistors, meansconnecting a different one of said resistors in series with each of thedevices, means for causing currents less than the predetermined currentsto flow through the devices, the current through any device decreasingto below such lesser currents in the event of a failure of such device,said current causing means causing currents to flow through each seriesresistor and device when such device is not operating, a plurality ofswitch means for selectively shorting each of said resistors forselectively increasing the current in each device to the predeterminedoperating current, an alarm, and switch means for energizing said alarmin response to such a decrease in the current through any of thedevices.

2. A circuit for detecting the electrical failure of any of a pluralityof electrical devices each of which requires a predetermined current tooperate comprising, in combination, means for causing currents less thanthe predetermined operating currents to flow through the devices, thecurrent through any device decreasing to below such lesser currents inthe event of a failure of such device, an alarm, first switch means forenergizing said alarm in response to such a decrease in the currentthrough any of the devices, a plurality of second switch means, each ofsaid second switch means having a predetermined voltage drop whenclosed, means connecting a different one of said second switch means inseries with each of the devices for selectively energizing the devices,and means connecting the junctions between each of the devices and theconnected second switch means to said first switch means for energizingsaid alarm.

3. In combination with a plurality of electrical devices, a circuit fordetecting the failure of any of said devices comprising a power source,a plurality of re sistors, each of said resistors connecting a differentone of said devices to said source whereby a predetermined low currentflows through each good device, said low current in each device beinglower than the current required to energize such device, means includinga plurality of isolation diodes for connecting the junctions betweensaid resistors and said devices to a common terminal whereby a firstvoltage appears on said terminal when all of said devices are good and asecond voltage appears on said terminal upon the failure of any of saiddevices, an alarm, and switch means connected to said terminal andresponsive to a change from said first voltage to said second voltagefor energizing said a nonconducting state when the second voltage isapplied to said control electrode, and a second transistor having aninput connected to said first transistor and an output connected to saidalarm, said second transistor conducting to energize said alarm whensaid first transistor is nonconducting.

5. The combination of claim 3 and including means for selectivelyenergizing each of said devices.

6. The combination of claim 5, wherein said means for selectivelyenergizing each of said devices includes a plurality of current limitingresistors, means connecting a different one of said current limitingresistors in series with each of said devices, a plurality of switches,and means connecting a different one of said switches in parallel witheach of said current limiting resistors whereby the closure of eachswitch energizes a different one of said devices.

7. The combination of claim 4, wherein said devices are incandescentlamps.

8. In combination with a plurality of electrical devices, a circuit fordetecting the failure of any of said devices comprising a power source,a plurality of first switch means, a predetermined current flowingthrough each of said first switch means when closed, means connectingeach of said devices and a different one of said first switch means inseries, means connecting all of said series connected devices and firstswitch means in parallel to said power source whereby said devices areselectively energized by selectively closing said first switch means, analarm, second switch means for energizing said alarm, means connectingsaid second switch means to the junctions between each of said devicesand said first switchmeans, said second switch means energizing saidalarm in response to a decrease in current through any of the junctionsbetween said devices and said first switch means to below thepredetermined current through said first switch means when such firstswitch means is closed.

9. The combination of claim 8, wherein said second switch means includesa plurality of electronic switches having input, output and controlelectrodes, means connecting said input and output electrodes to saidpower source, means connecting a different one of said controlelectrodes to each of the junctions between said devices and said firstswitch means, each of said electronic switch means changing state inresponse to a decrease in the current through the connected junction tobelowsaid predetermined-current, a common terminal, and means connectingsaid output electrodes to said common terminal whereby the currentthrough said common terminal is changed upon a failure of one or more ofsaid devices, and means responsive to the current through said terminalfor energizing said alarm.

10. A circuit for detecting the electrical failure of a device whichrequires a predetermined current to energize comprising, in combination,means for causing a current lower than the predetermined current to flowthrough the device, the current through the device decreasing to belowsuch lower current in the event of a failure of the device, an alarm,switch means for energizing said alarm in response to such a decrease inthe current through the device, a resistor, means connecting saidresistor in series with the device, the current to the device beingcaused to flow through said resistor when the device is not energized,and switch means for selectively shortmg said resistor to increase thecurrent in the device to the predetermined energizing current forselectively energizing said device.

11. In combination with a plurality of incandescent lamps, a circuit fordetecting the failure of any of said lamps comprising a power source, aplurality of first switch means, each of said first switch means havinga predetermined voltage drop when closed, means connecting each of saidlamps and a different one of said first switch means in series, meansconnecting all of said series connected lamps and first switch means inparallel to said power source whereby said lamps are selectivelyenergized by selectively closing said first switch means, an alarmincluding all of said lamps, second switch means for energizing saidalarm, and means connecting said second switch means to the junctionsbetween each of said lamps and said first switch means, said secondswitch means energizing said alarm in response to a decrease in voltageon any of said junctions between said lamps and said first switch meansto below the predetermined voltage drop of said first switch meanswhereby, when one or more of said lamps fail, the remaining good ones ofsaid lamps are all energized, I

12. The combination of claim 11, wherein said second switch meansincludesa plurality of electronic switches having input, output andcontrol electrodes, means connecting said input and output electrodes tosaid power source, means connecting a different one of said controlelectrodes to each of the junctions between said devices and said firstswitch means, each of said electronic switch means changing state inresponse to a decrease in the voltage on the connected junction to belowsaid predetermined voltage drop,- a common terminal, and meansconnecting said output electrodes to said common terminal whereby thevoltage on said common terminal is changed upon a failure of one or moreof said devices, and means responsive to the voltage on said terminalfor energizing said alarm.

1. A circuit for detecting the electrical failure of any of a pluralityof electrical devices each of which requires a predetermined current tooperate comprising, in combination, a plurality of resistors, meansconnecting a different one of said resistors in series with each of thedevices, means for causing currents less than the predetermined currentsto flow through the devices, the current through any device decreasingto below such lesser currents in the event of a failure of such device,said current causing means causing currents to flow through each seriesresistor and device when such device is not operating, a plurality ofswitch means for selectively shorting each of said resistors forselectively increasing the current in each device to the predeterminedoperating current, an alarm, and switch means for energizing said alarmin response to such a decrease in the current through any of thedevices.
 1. A circuit for detecting the electrical failure of any of aplurality of electrical devices each of which requires a predeterminedcurrent to operate comprising, in combination, a plurality of resistors,means connecting a different one of said resistors in series with eachof the devices, means for causing currents less than the predeterminedcurrents to flow through the devices, the current through any devicedecreasing to below such lesser currents in the event of a failure ofsuch device, said current causing means causing currents to flow througheach series resistor and device when such device is not operating, aplurality of switch means for selectively shorting each of saidresistors for selectively increasing the current in each device to thepredetermined operating current, an alarm, and switch means forenergizing said alarm in response to such a decrease in the currentthrough any of the devices.
 2. A circuit for detecting the electricalfailure of any of a plurality of electrical devices each of whichrequires a predetermined current to operate comprising, in combination,means for causing cUrrents less than the predetermined operatingcurrents to flow through the devices, the current through any devicedecreasing to below such lesser currents in the event of a failure ofsuch device, an alarm, first switch means for energizing said alarm inresponse to such a decrease in the current through any of the devices, aplurality of second switch means, each of said second switch meanshaving a predetermined voltage drop when closed, means connecting adifferent one of said second switch means in series with each of thedevices for selectively energizing the devices, and means connecting thejunctions between each of the devices and the connected second switchmeans to said first switch means for energizing said alarm.
 3. Incombination with a plurality of electrical devices, a circuit fordetecting the failure of any of said devices comprising a power source,a plurality of resistors, each of said resistors connecting a differentone of said devices to said source whereby a predetermined low currentflows through each good device, said low current in each device beinglower than the current required to energize such device, means includinga plurality of isolation diodes for connecting the junctions betweensaid resistors and said devices to a common terminal whereby a firstvoltage appears on said terminal when all of said devices are good and asecond voltage appears on said terminal upon the failure of any of saiddevices, an alarm, and switch means connected to said terminal andresponsive to a change from said first voltage to said second voltagefor energizing said alarm.
 4. The combination of claim 3, wherein saidswitch means comprises a first transistor having a control electrodeconnected to said terminal, means for biasing said first transistor intoa conducting state when the first voltage is applied to said controlelectrode and into a nonconducting state when the second voltage isapplied to said control electrode, and a second transistor having aninput connected to said first transistor and an output connected to saidalarm, said second transistor conducting to energize said alarm whensaid first transistor is nonconducting.
 5. The combination of claim 3and including means for selectively energizing each of said devices. 6.The combination of claim 5, wherein said means for selectivelyenergizing each of said devices includes a plurality of current limitingresistors, means connecting a different one of said current limitingresistors in series with each of said devices, a plurality of switches,and means connecting a different one of said switches in parallel witheach of said current limiting resistors whereby the closure of eachswitch energizes a different one of said devices.
 7. The combination ofclaim 4, wherein said devices are incandescent lamps.
 8. In combinationwith a plurality of electrical devices, a circuit for detecting thefailure of any of said devices comprising a power source, a plurality offirst switch means, a predetermined current flowing through each of saidfirst switch means when closed, means connecting each of said devicesand a different one of said first switch means in series, meansconnecting all of said series connected devices and first switch meansin parallel to said power source whereby said devices are selectivelyenergized by selectively closing said first switch means, an alarm,second switch means for energizing said alarm, means connecting saidsecond switch means to the junctions between each of said devices andsaid first switch means, said second switch means energizing said alarmin response to a decrease in current through any of the junctionsbetween said devices and said first switch means to below thepredetermined current through said first switch means when such firstswitch means is closed.
 9. The combination of claim 8, wherein saidsecond switch means includes a plurality of electronic switches havinginput, output and control electrodes, means connecting said input andoutpUt electrodes to said power source, means connecting a different oneof said control electrodes to each of the junctions between said devicesand said first switch means, each of said electronic switch meanschanging state in response to a decrease in the current through theconnected junction to below said predetermined current, a commonterminal, and means connecting said output electrodes to said commonterminal whereby the current through said common terminal is changedupon a failure of one or more of said devices, and means responsive tothe current through said terminal for energizing said alarm.
 10. Acircuit for detecting the electrical failure of a device which requiresa predetermined current to energize comprising, in combination, meansfor causing a current lower than the predetermined current to flowthrough the device, the current through the device decreasing to belowsuch lower current in the event of a failure of the device, an alarm,switch means for energizing said alarm in response to such a decrease inthe current through the device, a resistor, means connecting saidresistor in series with the device, the current to the device beingcaused to flow through said resistor when the device is not energized,and switch means for selectively shorting said resistor to increase thecurrent in the device to the predetermined energizing current forselectively energizing said device.
 11. In combination with a pluralityof incandescent lamps, a circuit for detecting the failure of any ofsaid lamps comprising a power source, a plurality of first switch means,each of said first switch means having a predetermined voltage drop whenclosed, means connecting each of said lamps and a different one of saidfirst switch means in series, means connecting all of said seriesconnected lamps and first switch means in parallel to said power sourcewhereby said lamps are selectively energized by selectively closing saidfirst switch means, an alarm including all of said lamps, second switchmeans for energizing said alarm, and means connecting said second switchmeans to the junctions between each of said lamps and said first switchmeans, said second switch means energizing said alarm in response to adecrease in voltage on any of said junctions between said lamps and saidfirst switch means to below the predetermined voltage drop of said firstswitch means whereby, when one or more of said lamps fail, the remaininggood ones of said lamps are all energized.